Adaptive signaling in multiple antenna systems

ABSTRACT

Briefly, in accordance with one embodiment of the invention, a wireless communication system may adaptively switch between a multiple input, multiple output mode and a spatial division, multiple access mode based at least in part on channel conditions and traffic conditions.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims the benefit of U.S. provisionalapplication Ser. No. 60/493,937, filed Aug. 8, 2003, entitled “AHIGH-THROUGHPUT WIRELESS NETWORK ARCHITECTURE, APPARATUS AND ASSOCIATEDMETHODS”.

BACKGROUND OF THE INVENTION

In a wireless communication network, multiple antennas may be utilizedat a transceiver in two possible ways: using a point-to-pointcommunication system or using a point-to-multipoint communicationsystem. A point-to-point communication system may be utilized tocommunicate with a single receiver to obtain higher signal quality andto provide a higher spectral efficiency. A point-to-multipointcommunication system may be utilized to communicate with multiplereceivers to obtain a higher signal quality and data throughput for eachreceiver. In such a multiple antenna wireless communication network, apoint-to-point communication system may be a multiple input, multipleoutput (MIMO) system, and a point-to-multipoint communication system maybe a spatial division, multiple access (SDMA) system.

DESCRIPTION OF THE DRAWING FIGURES

The subject matter regarded as the invention is particularly pointed outand distinctly claimed in the concluding portion of the specification.The invention, however, both as to organization and method of operation,together with objects, features, and advantages thereof, may best beunderstood by reference to the following detailed description when readwith the accompanying drawings in which:

FIG. 1 is a diagram of a wireless local area network communicationsystem in accordance with one embodiment of the present invention;

FIG. 2 is a diagram of a point-to-point system in accordance with oneembodiment of the present invention;

FIG. 3 is a diagram of a point-to-multipoint system in accordance withone embodiment of the present invention; and

FIG. 4 is a flow diagram of a method to adaptively switch between apoint-to-point link and a point-to-multipoint link in accordance with anembodiment of the present invention.

It will be appreciated that for simplicity and clarity of illustration,elements illustrated in the figures have not necessarily been drawn toscale. For example, the dimensions of some of the elements areexaggerated relative to other elements for clarity. Further, whereconsidered appropriate, reference numerals have been repeated among thefigures to indicate corresponding or analogous elements.

DETAILED DESCRIPTION

In the following detailed description, numerous specific details are setforth in order to provide a thorough understanding of the invention.However, it will be understood by those skilled in the art that thepresent invention may be practiced without these specific details. Inother instances, well-known methods, procedures, components and circuitshave not been described in detail so as not to obscure the presentinvention.

Some portions of the detailed description that follows are presented interms of algorithms and symbolic representations of operations on databits or binary digital signals within a computer memory. Thesealgorithmic descriptions and representations may be the techniques usedby those skilled in the data processing arts to convey the substance oftheir work to others skilled in the art. In some embodiments, suchalgorithms and data processing may include analog processing at basebandfrequencies, intermediate-frequencies (IF), or radio-frequencies (RF)implemented at least in part in hardware, in software, or in acombination thereof, although the scope of the invention is not limitedin this respect.

An algorithm is here, and generally, considered to be a self-consistentsequence of acts or operations leading to a desired result. Theseinclude physical manipulations of physical quantities. Usually, thoughnot necessarily, these quantities take the form of electrical ormagnetic signals capable of being stored, transferred, combined,compared, and otherwise manipulated. It has proven convenient at times,principally for reasons of common usage, to refer to these signals asbits, values, elements, symbols, characters, terms, numbers or the like.It should be understood, however, that all of these and similar termsare to be associated with the appropriate physical quantities and aremerely convenient labels applied to these quantities.

Unless specifically stated otherwise, as apparent from the followingdiscussions, it is appreciated that throughout the specificationdiscussions utilizing terms such as processing, computing, calculating,determining, or the like, refer to the action or processes of a computeror computing system, or similar electronic computing device, thatmanipulate or transform data represented as physical, such aselectronic, quantities within the registers or memories of the computingsystem into other data similarly represented as physical quantitieswithin the memories, registers or other such information storage,transmission or display devices of the computing system.

Embodiments of the present invention may include apparatuses forperforming the operations herein. This apparatus may be speciallyconstructed for the desired purposes, or it may comprise a generalpurpose computing device selectively activated or reconfigured by aprogram stored in the device. Such a program may be stored on a storagemedium, such as, but is not limited to, any type of disk includingfloppy disks, optical disks, CD-ROMs, magnetic-optical disks, read-onlymemories (ROMs), random access memories (RAMs), electricallyprogrammable read-only memories (EPROMs), electrically erasable andprogrammable read only memories (EEPROMs), flash memory, magnetic oroptical cards, or any other type of media suitable for storingelectronic instructions, and capable of being coupled to a system busfor a computing device.

The processes and displays presented herein are not inherently relatedto any particular computing device or other apparatus. Various generalpurpose systems may be used with programs in accordance with theteachings herein, or it may prove convenient to construct a morespecialized apparatus to perform the desired method. The desiredstructure for a variety of these systems will appear from thedescription below. In addition, embodiments of the present invention arenot described with reference to any particular programming language. Itwill be appreciated that a variety of programming languages may be usedto implement the teachings of the invention as described herein.

In the following description and claims, the terms coupled andconnected, along with their derivatives, may be used. In particularembodiments, connected may be used to indicate that two or more elementsare in direct physical or electrical contact with each other. Coupledmay mean that two or more elements are in direct physical or electricalcontact. However, coupled may also mean that two or more elements maynot be in direct contact with each other, but yet may still cooperate orinteract with each other.

It should be understood that embodiments of the present invention may beused in a variety of applications. Although the present invention is notlimited in this respect, the circuits disclosed herein may be used inmany apparatuses such as in the transmitters and receivers of a radiosystem. Radio systems intended to be included within the scope of thepresent invention include, by way of example only, wireless local areanetworks (WLAN) devices and wireless wide area network (WWAN) devicesincluding wireless network interface devices and network interface cards(NICs), base stations, access points (APs), gateways, bridges, hubs,cellular radiotelephone communication systems, satellite communicationsystems, two-way radio communication systems, one-way pagers, two-waypagers, personal communication systems (PCS), personal computers (PCs),personal digital assistants (PDAs), sensor networks, personal areanetworks (PANs) and the like, although the scope of the invention is notlimited in this respect.

Types of wireless communication systems intended to be within the scopeof the present invention include, although not limited to, WirelessLocal Area Network (WLAN), Wireless Wide Area Network (WWAN), CodeDivision Multiple Access (CDMA) cellular radiotelephone communicationsystems, Global System for Mobile Communications (GSM) cellularradiotelephone systems, North American Digital Cellular (NADC) cellularradiotelephone systems, Time Division Multiple Access (TDMA) systems,Extended-TDMA (E-TDMA) cellular radiotelephone systems, third generation(3G) systems like Wide-band CDMA (WCDMA), CDMA-2000, and the like,although the scope of the invention is not limited in this respect.

Referring now to FIG. 1, a wireless local area network communicationsystem in accordance with one embodiment of the present invention willbe discussed. In the WLAN communications system 100 shown in FIG. 1, amobile unit 110 may include a wireless transceiver 112 to couple to anantenna 118 and to a processor 114 to provide baseband and media accesscontrol (MAC) processing functions. Processor 114 in one embodiment maycomprise a single processor, or alternatively may comprise a basebandprocessor and an applications processor, although the scope of theinvention is not limited in this respect. Processor 114 may couple to amemory 116 which may include volatile memory such as DRAM, non-volatilememory such as flash memory, or alternatively may include other types ofstorage such as a hard disk drive, although the scope of the inventionis not limited in this respect. Some portion or all of memory 116 may beincluded on the same integrated circuit as processor 114, oralternatively some portion or all of memory 116 may be disposed on anintegrated circuit or other medium, for example a hard disk drive, thatis external to the integrated circuit of processor 114, although thescope of the invention is not limited in this respect.

Mobile unit 110 may communicate with access point 122 via wirelesscommunication link 132, where access point 122 may include at least oneantenna 120. In an alternative embodiment, access point 122 andoptionally mobile unit 110 may include two or more antennas, for exampleto provide a spatial division multiple access (SDMA) system or amultiple input, multiple output (MIMO) system, although the scope of theinvention is not limited in this respect. Access point 122 may couplewith network 130 so that mobile unit 110 may communicate with network130, including devices coupled to network 130, by communicating withaccess point 122 via wireless communication link 132. Network 130 mayinclude a public network such as a telephone network or the Internet, oralternatively network 130 may include a private network such as anintranet, or a combination of a public and a private network, althoughthe scope of the invention is not limited in this respect. Communicationbetween mobile unit 110 and access point 122 may be implemented via awireless local area network (WLAN), for example a network compliant witha an Institute of Electrical and Electronics Engineers (IEEE) standardsuch as IEEE 802.11a, IEEE 802.11b, IEEE 802.1 μg, IEEE 802.11n,HiperLAN-II, and so on, although the scope of the invention is notlimited in this respect. In another embodiment, communication betweenmobile unit 110 and access point 122 may be at least partiallyimplemented via a cellular communication network compliant with a 3GPPstandard, although the scope of the invention is not limited in thisrespect.

Referring now to FIG. 2, a diagram of a multiple input, multiple outputsystem in accordance with an embodiment of the present invention will bediscussed. The multiple input, multiple output (MIMO) system 200 shownin FIG. 2 illustrates a MIMO link 222 between a transmitting transceiver210 that may include a first antenna 212 up to M_(T) 214 transmitantennas, and a receiving transceiver 216 that may include a firstantenna 218 and up to M_(R) 220 receive antennas. The MIMO system 200 ofFIG. 2 may be analogous to the wireless local area network system 100 ofFIG. 1 where transmitting transceiver 210 may correspond to access point122 and receiving transceiver 216 may correspond to mobile unit 110,although the scope of the invention is not limited in this respect. Thecomplex baseband channel at the i^(th) receive antenna from the i^(th)transmit antenna may be defined as the [r,t] entry in the M_(R)×M_(T)channel matrix H. The channel matrix H may be defined as:$H = \begin{bmatrix}h_{11} & \cdots & h_{1M_{T}} \\\quad & ⋰ & \quad \\h_{M_{R}1} & \cdots & h_{M_{R}M_{T}}\end{bmatrix}$In one embodiment of the invention, the MIMO channel may correspond to asingle carrier system. For example, in a multicarrier orthogonalfrequency division multiplexing (OFDM) system, the channel matrix maycorrespond to the MIMO channel on one frequency tone, and may bedifferent from tone to tone, although the scope of the invention is notlimited in this respect.

In general, the channel matrix may be a random variable that may dependon the location of transmitting transceiver 210 and receivingtransceiver 216, the placement of antennas 212 to 214 and 218 to 220,the carrier frequency, and the scattering environment aroundtransmitting transceiver 210 and receiving transceiver 216. The channelmatrix may vary with time depending on the rate at which the environmentis changing or the rate at which transmitting transceiver 210 andreceiving transceiver 216 may be moving. In accordance with oneembodiment of the present invention, the channel may be reliablyestimated at receiving transceiver 216.

To obtain channel knowledge at the transmitting transceiver 210, such asin a time division duplex (TDD) systems, the channel matrix H may bedetermined based on the channel estimated on the reverse link from thereceiving transceiver 216 to the transmitting transceiver 210. In afrequency division duplex (FDD) system, the channel from thetransmitting transceiver 210 to the receiving transceiver 216 may behighly uncorrelated with the channel in the reverse direction, so activefeedback of the channel from the receiving transceiver 216 to thetransmitting transceiver 210 may be utilized. Even in a TDD system, ifthe channel changes during the time between receiving and transmitting,or if the calibration on the transmit and receive radio-frequency (RF)chains is not sufficiently accurate, active feedback of the channel fromthe receiving transceiver 216 to the transmitting transceiver 210 may beutilized as well.

In accordance with one embodiment of the present invention, one of twoways to signal over a MIMO link may be utilized, depending on the degreeof channel knowledge available at the transmitting transceiver 210:open-loop signaling where the transmitting transceiver 210 has noknowledge of the channel; and closed loop signaling where thetransmitting transceiver 210 may have partial or complete informationabout the channel matrix H, for example the value of H, or alternativelystatistics of H such as the correlation E HH*.

In accordance with one embodiment of the invention, both open loopsignaling and transmitter-trained signaling techniques may be used toimprove signal quality, for example the signal-to-noise ratio (SNR), atthe receiving transceiver 216 regardless of the actual channelrealization. Either of these techniques may be utilized to increasespectral efficiency as measured in bits per second per hertz, andperformance may be a function of the channel realization H. Inaccordance with one particular embodiment of the invention, depending onthe physics of the propagation environment, H may be conditioned tosupport a higher spectral efficiency. The condition number of H, κ(H)may be defined using the singular value decomposition of H=UΣV* asκ=σ₁/σ_(M), where σ₁ is the strongest singular value and σ_(M) is theweakest singular value of H and M=minimum(M_(T),M_(R)). $\begin{matrix}{H = {\begin{bmatrix}h_{11} & \cdots & h_{1M_{T}} \\\quad & ⋰ & \quad \\h_{M_{R}1} & \cdots & h_{M_{R}M_{T}}\end{bmatrix} = {U\quad\Sigma\quad V^{*}}}} \\{\Sigma = \begin{bmatrix}\sigma_{1} & \quad & \quad & \quad \\\quad & ⋰ & \quad & \quad \\\quad & \quad & \sigma_{M} & \quad \\\quad & \quad & \quad & 0\end{bmatrix}} \\{\kappa = \frac{\sigma_{1}}{\sigma_{M}}}\end{matrix}$Both the open-loop and transmitter-trained MIMO techniques may provideoptimal performance when κ=1, that is when most or all the singularvalues are equal. In the event the singular values are unequal, thenκ>1. The larger the value of κ, the more ill-conditioned the channel Hmay be, and the smaller the improvement in spectral efficiency providedby the M_(R)×M_(T) point-to-point MIMO link.

Referring now to FIG. 3, a spatial division multiple access system inaccordance with an embodiment of the present invention will bediscussed. As shown in FIG. 3, spatial division multiple access (SDMA)system 300 may include a transmitting transceiver 310, a first receivingtransceiver 312, and up to a Uth receiving transceiver 314. In oneembodiment of the invention, transmitter 310 may correspond to accesspoint 122 of FIG. 1 and to transmitting transceiver 210 of FIG. 2.Likewise, receivers 312 to 314 may correspond to mobile unit 110 of FIG.1 and to receiving transceiver 216 of FIG. 2, although the scope of theinvention is not limited in this respect. In contrast to MIMO system200, which in one embodiment may be a physical (PHY) layer technique,SDMA system 300 may be a media access control (MAC) layer technique thatutilizes multiple antennas to simultaneously serve multiple users, suchas U number of receivers. The SDMA system 300 may provide apoint-to-multipoint SDMA link 316 between transmitting transceiver 310,which may have a first antenna 318 and up to M_(T) 320 transmitantennas, and U receivers 312-314 having first antennas 322 and 326 upto M_(R) 324 and 328 receive antennas, although the scope of theinvention is not limited in this respect. The M_(R)×M_(T) complexbaseband channel at the U^(th) receiver may be labeled as H_(u) whichmay be specified as follows: $H_{U} = \begin{bmatrix}h_{11} & \cdots & h_{1M_{T}} \\\quad & ⋰ & \quad \\h_{M_{R}1} & \cdots & h_{M_{R}M_{T}}\end{bmatrix}_{(U)}$Since different receiving transceivers 312 to 314 may be located atdifferent physical locations, the channels of transceivers 312 to 314are likely to be highly uncorrelated. As in MIMO system 200, these Uchannels may correspond to a single carrier system, and may beinterpreted for example as the channels on a single tone of an OFDMsystem, although the scope of the invention is not limited in thisrespect. Furthermore, although the SDMA system 300 of FIG. 3 shows M_(R)receive antennas on the transceivers 312 to 314, the number of receiveantennas may vary from user 312 to user 314, although the scope of theinvention is not limited in this respect. In such a case, M_(R) may beinterpreted as a maximum, or near maximum, number of receive antennas onthe receivers 312 to 314, although the scope of the invention is notlimited in this respect.

The multi-user channel at transmitter 310 may consist of a stacking ofall the single-user channels as shown by:H=[H₁ . . . H_(U)]For comparison to SDMA link 316, MIMO link 222 as shown in FIG. 2 may beinterpreted in one case as a point-to-multipoint link where data may beconcurrently encoded over multiple transmit antennas and concurrentlydecoded at multiple receive antennas, and may be considered ascorresponding to virtual multiple SDMA users, and as a result mayprovide a higher spectral efficiency over the MIMO link, although thescope of the invention is not limited in this respect.

Taking into consideration the MAC layer traffic characteristics, anetwork with SDMA link 316 in one embodiment may outperform a networkwith independent MIMO links 222 in terms of aggregate throughput. Such aresult may be especially appreciable in network hot-spots, wheremultiple MIMO users may encounter frequent collisions before they areable to access the carrier sense multiple access (CSMA) medium. As aresult of such collisions, users may have to use random back off andwait before transmitting again, which may increase the latency perpacket. In contrast, SDMA users may be served simultaneously withoutincreasing the probability of collision, thereby resulting in highernetwork throughput and lower latency, although the scope of theinvention is not limited in this respect.

Referring now to FIG. 4, a flow diagram of a method for adaptivesignaling over multiple antennas by switching between a multiple input,multiple output mode and a spatial division, multiple access mode inaccordance with an embodiment of the invention will be discussed. Asshown in FIG. 4, method 400 may be utilized in a wireless local areanetwork system such as WLAN system 100 of FIG. 1 to allow WLAN system100 to switch between a MIMO mode and an SDMA mode. In one embodiment ofthe invention, when method 400 switches to a MIMO mode, MIMO operationmay be performed using open-loop signaling although the scope of theinvention is not limited in this respect. In an alternative embodiment,either one or both MIMO and SDMA operation may be performed usingclosed-loop signaling, for example when the channel is stationary ornearly stationary, although the scope of the invention is not limited inthis respect.

In one embodiment of the invention, method 400 may be executed by accesspoint 122 of FIG. 1, for example as instructions executed by basebandprocessor 126 and stored in memory 128, although the scope of theinvention is not limited in this respect. In such an embodiment, accesspoint 122 may function as transmitting transceiver 210 of FIG. 2 whileoperating in a MIMO mode, and may function as transmitting transceiver310 of FIG. 3 while operating in an SDMA mode, although the scope of theinvention is not limited in this respect. At block 410, access point 122may estimate and store the MIMO channel matrices of up to U receivers. Adetermination may be made at block 412 whether to operate in a MIMO modeor in an SDMA mode. In the event most of the U channels areill-conditioned, method 400 may proceed along branch 414 to block 416where operation may occur using point-to-multipoint SDMA to the U usersconcurrently.

While operating in an SDMA mode, at block 418 access point 122 mayobserve the performance of the PHY layer at the receivers 312 to 314,and the MAC layer at transmitting transceiver 310. A determination maybe made at block 420 whether to continue in an SDMA mode or to switch toa MIMO mode, based on the performance of the PHY and MAC layers observedat block 418. In one embodiment of the invention, good performance ofthe PHY layer may be defined as operating at a higher data rate, at ahigher signal-to-noise ratio (SNR), at a lower bit error rate (BER), andat a higher spectral efficiency, although the scope of the invention isnot limited in this respect. In one embodiment of the invention, goodperformance of the MAC layer may be defined as operating with a lowerlatency and at higher throughput with relatively few retransmits,although the scope of the invention is not limited in this respect. Inthe event of good PHY layer and MAC layer performance, method 400 maycontinue along branch 422 and continue to operate in an SDMA mode atblock 416, although the scope of the invention is not limited in thisrespect.

In the event of poor PHY performance, for example in the event of alower spectral efficiency per user, method 400 may switch from an SDMAmode to a MIMO mode by executing along branch 424. Such a switch from anSDMA mode to a MIMO mode may occur even where the performance of the MAClayer may be considered good, although the scope of the invention is notlimited in this respect. In an alternative embodiment, one or more ofreceivers 312 to 314 may require a higher throughput than others, forexample a receiver may require a higher quality of service (QoS), whichmay be provided by serving fewer simultaneous users at higher spectralefficiencies per user. In such a case, method 400 may switch from anSDMA mode to a MIMO mode by executing along branch 424 to operate in aMIMO mode at block 428, although the scope of the invention is notlimited in this respect.

In the event at block 412 it is determined that most of the U channelsare well conditioned, then method 400 may execute in a point-to-pointMIMO mode at block 428 in which transmitting transceiver 210 maycommunicate with one receiving transceiver 216 at a time. Whileoperating in a MIMO mode, the PHY layer performance of the receivingtransceivers 216 and the MAC layer performance of the transmittingtransceiver 210 may be observed at block 430. A determination may bemade at block 432 whether to continue in a MIMO mode or to switch to anSDMA mode. If the PHY layer performance and the MAC layer performanceare good, then method 400 may execute along branch 434 and continueoperating in a MIMO mode at block 428, although the scope of theinvention is not limited in this respect.

In the event the MAC layer performance is poor, for example where thereis a higher number of receiving transceivers resulting in a highernumber of collisions, then method 400 may switch from a MIMO mode to anSDMA mode by executing along branch 436 to operate in an SDMA mode atblock 416. Such a switch from a MIMO mode to an SDMA mode may occur evenwhere the performance of the PHY is good, although the scope of theinvention is not limited in this respect. In accordance with oneembodiment of the invention, method 400 may adapt WLAN system 100 toprovide a higher aggregate network throughput and a lower averagelatency in response to channel conditions and traffic conditions,although the scope of the invention is not limited in this respect. Inone embodiment of the invention, while operating in a MIMO mode, channelconditions may occur where signal quality may be improved, but spectralefficiency may not be improved. In such a case, it may not be efficientto use all of the transmit antennas 212 to 214 for one MIMO receivingtransceiver 216 at a time, and as a result it may be more efficient toserve multiple users simultaneously. In such a case, method 400 mayswitch from a MIMO mode to an SDMA mode by executing along branch 436 tooperate in an SDMA mode at block 416, although the scope of theinvention is not limited in this respect.

In accordance with one embodiment of the invention, while executing themethod 400 as shown, WLAN system 100 may obtain updated channelestimates over time. Such updated channel estimates may be obtained toaccommodate for changes in the physical environment, for example frommovement of the users or from additional users entering or leaving theenvironment of WLAN system 100, that may affect the performance of thePHY layer. Such changes in the physical environment may affect theobservations made at block 418 and at block 430 so that method 400 mayswitch between a MIMO mode and an SDMA mode in accordance with changesin the channel, although the scope of the invention is not limited inthis respect.

In one embodiment of the invention, point-to-point operation in a MIMOmode and point-to-multipoint operation in an SDMA mode as embodied bymethod 400 may analogously extend to higher bandwidth channels as wellin accordance with the present invention. For example, a point-to-pointhigher bandwidth channel system may serve one user at a time at a higherbandwidth, whereas a point-to-multipoint higher bandwidth channel systemmay serve multiple lower channel users at a time at lower bandwidths forthe users. In accordance with the present invention, adaptation betweenpoint-to-point channel bonding systems and point-to-multipoint channelbonding systems may be performed in a manner substantially similar asmethod 400, although the scope of the invention is not limited in thisrespect.

Although the invention has been described with a certain degree ofparticularity, it should be recognized that elements thereof may bealtered by persons skilled in the art without departing from the spiritand scope of the invention. It is believed that the adaptive signalingin multiple antenna systems of the present invention and many of itsattendant advantages will be understood by the forgoing description, andit will be apparent that various changes may be made in the form,construction and arrangement of the components thereof without departingfrom the scope and spirit of the invention or without sacrificing all ofits material advantages, the form herein before described being merelyan explanatory embodiment thereof, and further without providingsubstantial change thereto. It is the intention of the claims toencompass and include such changes.

1. A method, comprising: operating in a multiple input, multiple outputmode; and in the event of a predetermined condition, operating in aspatial division, multiple access mode.
 2. A method as claimed in claim1, wherein the predetermined condition includes a latency valueexceeding a predetermined value.
 3. A method as claimed in claim 1,wherein the predetermined condition includes a throughput value beingbelow a predetermined value.
 4. A method as claimed in claim 1, whereinthe predetermined condition includes a number of collisions exceeding apredetermined value.
 5. A method as claimed in claim 1, wherein thepredetermined condition includes a desired higher spectral efficiency.6. A method as claimed in claim 1, wherein the predetermined conditionincludes a number of receivers exceeding a predetermined value.
 7. Amethod, comprising: operating in a spatial division, multiple accessmode; and in the event of a predetermined condition, operating in amultiple input, multiple output mode.
 8. A method as claimed in claim 5,wherein the predetermined condition includes a spectral efficiency peruser being below a predetermined value.
 9. A method as claimed in claim5, wherein the predetermined condition includes a data rate being belowa predetermined value.
 10. A method as claimed in claim 5, wherein thepredetermined condition includes a desired higher data rate for at leastone user.
 11. A method as claimed in claim 5, wherein the predeterminedcondition includes a desired higher quality of service for at least oneuser.
 12. A method, comprising: estimating a channel matrix for at leastone or more receivers; in the event the channels are well-conditioned,operating in a multiple input, multiple output mode; and otherwiseoperating in a spatial division, multiple access mode.
 13. A method asclaimed in claim 12, further comprising, while operating in a multipleinput, multiple output mode, observing a media access layer performanceat transmitter, and in the event of poor media access layer performance,switching to a spatial division, multiple access mode.
 14. A method asclaimed in claim 12, further comprising, while operating in a spatialdivision, multiple access mode, observing physical layer performance ofthe at least one or more receivers, and in the event of poor physicallayer performance, switching to a multiple input, multiple output mode.15. An article, comprising: a storage medium having stored thereoninstructions that, when executed by a computing platform, result inadaptive switching between a multiple input, multiple output mode and aspatial division, multiple access mode by: operating in a multipleinput, multiple output mode; and in the event of a predeterminedcondition, operating in a spatial division, multiple access mode.
 16. Anarticle as claimed in claim 15, wherein the predetermined conditionincludes a latency value exceeding a predetermined value.
 17. An articleas claimed in claim 15, wherein the predetermined condition includes athroughput value being below a predetermined value.
 18. An article asclaimed in claim 15, wherein the predetermined condition includes anumber of collisions exceeding a predetermined value.
 19. An article asclaimed in claim 15, wherein the predetermined condition includes adesired higher spectral efficiency.
 20. An article as claimed in claim15, wherein the predetermined condition includes a number of receiversexceeding a predetermined value.
 21. An article, comprising: a storagemedium having stored thereon instructions that, when executed by acomputing platform, result in adaptive switching between a multipleinput, multiple output mode and a spatial division, multiple access modeby: operating in a spatial division, multiple access mode; and in theevent of a predetermined condition, operating in a multiple input,multiple output mode.
 22. An article as claimed in claim 21, wherein thepredetermined condition includes a spectral efficiency per user beingbelow a predetermined value.
 23. An article as claimed in claim 21,wherein the predetermined condition includes a data rate being below apredetermined value.
 24. An article as claimed in claim 21, wherein thepredetermined condition includes a desired higher data rate for at leastone user.
 25. An article as claimed in claim 21, wherein thepredetermined condition includes a desired higher quality of service forat least one user.
 26. An article, comprising: a storage medium havingstored thereon instructions that, when executed by a computing platform,result in adaptive switching between a multiple input, multiple outputmode and a spatial division, multiple access mode by: estimating achannel matrix for at least one or more receivers; in the event thechannels are well-conditioned, operating in a multiple input, multipleoutput mode; and otherwise operating in a spatial division, multipleaccess mode.
 27. An article as claimed in claim 26, wherein theinstructions, when executed, further result in adaptive switchingbetween a multiple input, multiple output mode and a spatial division,multiple access mode by, while operating in a multiple input, multipleoutput mode, observing a media access layer performance at transmitter,and in the event of poor media access layer performance, switching to aspatial division, multiple access mode.
 28. A method as claimed in claim27, wherein the instructions, when executed, further result in adaptiveswitching between a multiple input, multiple output mode and a spatialdivision, multiple access mode by, while operating in a spatialdivision, multiple access mode, observing physical layer performance ofthe at least one or more receivers, and in the event of poor physicallayer performance, switching to a multiple input, multiple output mode.29. An apparatus, comprising: a transceiver; at least two or moreomnidirectional antennas to couple to said transceiver; and a basebandprocessor to couple to said transceiver, wherein said baseband processorand said transceiver switch from a multiple input, multiple output modeto a spatial division, multiple access mode under a first condition, andswitch from a spatial division, multiple access mode to a multipleinput, multiple output mode under a second condition.
 30. An apparatusas claimed in claim 29, wherein the first condition includes at leastone of a higher latency, a lower throughput, a higher number ofretransmits, and a higher number of receivers.
 31. An apparatus asclaimed in claim 29, wherein the second condition includes at least oneof a lower signal-to-noise ratio, a higher bit error rate, a lowerspectral efficiency, a desired higher data rate for at least onereceiver, a desired higher quality of service for at least one receiver,and a lower number of receivers.